JPS5960274A - Metal detector - Google Patents

Abstract

PURPOSE:To enable a highly sensitive detection of both ferrous and non-ferrous metal by driving a transmission coil with the time division of two frequencies. CONSTITUTION:When an object W to be inspected containing iron passes as shown by the arrow, the balance of induced voltage of a receiving coil 14, particularly the balance of the amplitude breaks and a differential signal of a waveform C is outputted. A synchronous detection signal of a waveform D is supplied to a synchronous detector 16 through a switch 21 and a differential signal C' is detected synchronously by synchronous detection signals eF and eS varied in the phase at each Tl during the period Th to obtain a synchronous detection signal of a waveform E. The waveform E is time divided with a switch 22 to be inputted separately into BPF17a and 17b and the input waveform gives a detection signal having frequency fh and fl components as shown by F and G. The signal F is zero in the mean value and a discriminator circuit 18a fails to operate while the signal G is Ef in the mean value and the discriminator 18b operates to output a signal indicating mixing-in of iron.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a metal detection device for detecting whether metal is mixed in an object to be inspected (particularly food) being conveyed by a conveyor or the like.

First, the general invasion of a conventionally used metal detection device will be explained with reference to FIG.

In this figure, 1 is an oscillator, 2 is a transmitting coil connected to the oscillator 1, 3a, 3b is a receiving coil arranged opposite to the sending coil 2, and this receiving coil 3a is arranged opposite to the sending coil 2. , 3b are placed in the alternating magnetic field of the transmission filter 2, and arranged so that their magnetic force pairs are equally intersected.

4a and 4b are induced voltages e of the receiving coils 3a and 3b,
Indicates the adjustment volume for the phase and amplitude of +e2, and by adjusting the adjustment voltages 4a and 4b, the differential induced voltage of the receiving coils 3a and 3b is set to 6, -mi, and 20. . 5 is differential induced voltage e+ e2
6a and 6b are synchronous detectors that detect ferrous and non-ferrous metals, 7a and 7b are low-pass filters, and Qa and 8b are discrimination circuits. Note that 9a and 9b indicate first and second phase shifters that form synchronizing signals to be supplied to the synchronous detectors 6a and 6b.

The metal detection device having the above configuration includes a transmitting coil 2. When the inspected object W passes between the coils 3a and 31 and the inspected object WK metal is mixed in, it is determined by the type of metal (ferrous or non-ferrous) VC + J38a, B1
) Detection 1 occurs.

To explain this point using the vector diagrams in Figure i22 (a) and (b), normally, the induced voltage el+ of the transmission coils 3a and 3b is
At the input terminal of amplifier 5;! + eJ is set to 20, but the test object W containing iron
When passing from the direction of the arrow, the electromotive force 61 of the receiving coil 3a increases by a; VC, as shown in FIG. 2 (al), and then the induced voltage 2 of the receiving coil 3b increases.

Therefore, the differential induced voltages b', -mi, and 2eot are input to the synchronous detector 6a&, and only the same-phase synchronous detection signal 4 is supplied to the synchronous detector 6a. Detected by 4. Ru.

On the other hand, when the inspected object W containing non-ferrous metals (stainless steel, aluminum, etc.) passes through, eddy currents flow in the non-ferrous metals under the influence of the oscillator 10-cross b1ε magnetic field.

Then, due to the influence of this eddy current, the receiving coil 3a.

The phase of the induced voltage el r e@ of 3b changes.

That is, 1. lJ (as shown in Figure 2(b), when the phase of the induced voltage in the receiving coil 3a changes to Jl, the difference m induced voltage e I @ 2 = e, s has a phase of approximately 90° as shown in the figure). Therefore, the non-ferrous metal is detected by phase detection in the synchronous detector 6b, which is supplied with the signal for synchronous detection shown in . (When the induced paper pressure ``e2'' changes to v2, it can be detected for the same reason.) The metal detection operation of the object W to be inspected has been briefly explained above. Consider frequency and metal detection sensitivity.

Figure 3 shows iron (Fe) when the frequency of oscillator 1 is changed.
This shows the sensitivity index for non-ferrous metals (stainless steel (SUS)) and products (sausages).
It shows a constant sensitivity index and the detection sensitivity does not change, but since the eddy current loss of non-ferrous (SUS) and products is proportional to the square of the frequency, as the frequency increases, the sensitivity index becomes higher than that of iron. It can be seen that in order to detect (Fe), it is better to use a signal (f,) with a lower frequency.

On the other hand, when comparing products with nonferrous steel (SUS), as shown in Figure 4, the sensitivity index changes depending on the phase detected by detector No. 18, and for SUS it reaches a maximum around 00, and qo't (
The lowest point is near Jl, but in the case of products, the lowest point exists at a phase different from that of nonferrous steel (SUS) due to its physical properties, as shown by Hamori.

And, the phase difference between the non-ferrous metal (SUS) and the point where the product has the lowest sensitivity is the lower frequency f, K.
You can see that the larger the button, the larger the button.

In other words, the sensitivity difference between non-ferrous metal (SUS) and the product is between a and b when the phase difference of the synchronous detection signal is selected to be around 80 degrees at low frequency fl, but at low frequency fh a
Since the sensitivity difference (phase) of '-b' can be selected at approximately 70°, it can be seen that a higher frequency is more advantageous when distinguishing between products and non-ferrous metals.

According to the present invention, the metal detection device is operated in a time-sharing manner so that the sensitivity index for both ferrous and non-ferrous metals is improved based on the data analysis results as described above.

Hereinafter, an embodiment of the metal detection device of the present invention will be explained with reference to the block circuit diagram of FIG. 5.

In this circuit, 1o is an oscillator, 11 is a frequency divider of l'E1 of l//rI, and 12 is a second frequency divider of l/m. The first frequency divider 11 has a frequency f of 90
.

forming a lower frequency it, the second frequency divider 12
As will be described later, the drive pulses (timing pulses) for the time-division switching switches 20 to 22 are formed.

Reference numerals 13 and 14 indicate the above-mentioned transmitting 46 filter and receiving filter, and 15 is an amplifier that outputs the differential induced voltage of the receiving coil 14.

1G is a synchronous detector, 17a and 17b are band pass filters, and 18a and 18b are discrimination circuits.

The 1111th synchronous detector 16 includes two phase shifters 19a.

Two signals for synchronous detection are supplied from 19b.

Next, the operation of the above embodiment will be explained with reference to the waveform diagram of FIG.

) The signal waveform (B) of the #3 vibrator 10 is set to a frequency fh at which the difference in sensitivity index between the non-ferrous metal (SUS) and the product becomes large, as explained using the data in FIG. These frequencies are stepped down by the first frequency divider 11, and are divided so that j' in Fig. 3 is the frequency f at which the difference in sensitivity index between ferrous and non-ferrous (products) becomes large as explained in the previous section. Go around. FIG. 6(A) shows the waveform of the frequency ft when the frequency division ratio is +/2.

These two frequency signals Dfh+fL are sent to the second frequency divider 1.
18 coil 13 via a switch 20 which is switched by a timing pulse (H) formed by
Therefore, in the supplied waveform (C), the frequency components of fh and fz are alternately output at the period T0 of the timing pulse (H).

When the standalone W to be tested is not σ between the transmitting coil 13 and the receiving coil 140, the induced 'Ichikawa' difference output of the receiving coil 14 is adjusted to be zero, so the synchronous detector 16 No detection error occurs.

Now, in this state, the object to be inspected W containing iron passes through the receiving coil 14 as shown by the arrow. Since the balance between the induced voltage and the 1L pressure - 1° and 2 and the load destroys the equality of amplitude, a difference signal as shown in the waveform (C) of FIG. 6 is output.

Since the synchronous detector 16 is supplied with a signal for synchronous detection as shown in the waveform (D) in FIG. Then, synchronous detection is performed using signals i and 8 for synchronous detection having different phases for each period TL, and a synchronous detection signal as shown in the waveform (E) in FIG. 6 is finally obtained.

This (E) waveform is further time-divided by the switch 22 and inputted separately to the bandpass filters 17a and 17b, so that each input waveform is (F).

As shown in (G), it becomes a detected signal with frequencies f and l f components.

Since the fh acid component detection signal (F) has an average value of zero, the discriminator circuit 18a does not operate, and the 11 component detection signal (G) has an average value of El, so the discriminator circuit 18a does not operate.
8b operates and outputs a signal indicating that iron is mixed.

When non-ferrous metal (SUS) is mixed in the object W to be inspected, the same operation is performed, but in this case, as explained in the data diagram of FIG.
The effect of eddy current is small, frequency f, acid component reception 1g.
It is considered that there is almost no output tl due to cancellation within the coil 14.

Therefore, in FIG. 6, the vortex 11t only during the period T.
The current causes a difference output in the induced voltage of the receiving coil 14, but since this difference output is caused by phase fluctuation, the sensitivity index of the product is the lowest as explained in the data diagram of Figure 4. FIG. 7 shows another embodiment of the present invention in which a changeover switch is also provided in the transmitter/receiver (R fill) for time-division operation.

That is, two transmitting coils 13a and 13b are formed,
With the switches 23 and 24VC, only the transmitting coil 13a is used when the frequency is fh, and the frequency is fh.

At this time, the transmitting coils 13a and 13b are controlled to be connected in series, and the timing pulse 0 () is used to connect the transmitting coils 13a and 13b in series, and to insert the capacitors c1 and c2 for the frequency fb r r, by switching them with the switch 25. control.

The receiving coils 14a and 14b are further divided into two parts 14a1.
+14a2+ and 14b+, 14bz, young 1
．． is controlled by switches 26 and 27 so that the frequency is in series with fL.

In this way, the impedance matching of the transmitting and receiving coils can be controlled to be optimal for both the two frequencies fb and fL, so that (1-) the detection sensitivity of the K metal detection device can be improved.

Incidentally, since the phase at which the lowest sensitivity is obtained differs depending on the type of product, a signal different from the frequency fh may be used at that time.

As explained above, the metal detection device of the present invention detects the transmitting/receiving coil with signals of at least two frequencies fh and fl, detects iron at the lower frequency ft that does not affect the product, and detects non-ferrous metals at the lower frequency ft that does not affect the product. For metal detection, a high frequency f with a crystalline sensitivity index and good phase discrimination is required.
Since I set K to detect at h, is it ferrous or non-ferrous? It is possible to detect signals with two frequencies that can only be detected with % sensitivity in a time-sharing manner.
, 11, so gold λ1 (・1 eye 1''
Most of the parts of the file direction (I'4) are the same as before (
It has the advantage of being able to achieve I'□S.

・Figure 15 curve l1i1 simple situation rAl21 is a block diagram of a conventional metal detection device, and Figures 2 (a) and (b) are vectors used to detect ferrous and non-ferrous metals11. ．． Ic: (t4, Figure 4 is a theta diagram for explaining the sensitivity index, supply frequency range, and phase relationship for iron, nonferrous metals (S tJ S) and products,
FIG. 5 is a block diagram of a metal detection device showing one embodiment of this invention, FIG. 6 is a main waveform diagram of FIG. 5, and FIG. 7 is a switching diagram of a transmitting/receiving coil showing another embodiment of this invention. be.

In the figure, 10 is an oscillator, 11 is a first branch, and 12 is a second
13 is a transmitting coil, 14 is a receiving δ coil, 1
5 is a type width detector, 16 is a synchronous detector, 20 to 27 are 11, "
shows a switch that operates with split timing pulses.

Figure 1 1+ Figure 2 (a)(b) 366- Figure 3 f+ong-Figure 4 Phase □

Claims (1)

[Claims] A transmitting coil to which signals of at least two different frequencies (fh, f,) are supplied in a time-division manner, two receiving coils provided opposite to the transmitting coil, and these two receiving coils. Amplification to form the difference output of the induced voltage of:
and a synchronous detector for detecting the output of the intensifier, and the two synchronous detection signals obtained by phase-shifting the signals of the two frequencies (f), fx) are 1'? A metal detection device having a q1 feature of supplying the synchronous detector by f division.